1. Field of the Invention
The present invention relates to a switching regulated power supply apparatus. More specifically, the present invention relates to an improvement in a switching regulated power supply apparatus wherein a direct current voltage obtained through rectification of a commercial alternate current power supply input is applied to a direct current/alternate current converting circuit implemented by a series connection of a switching device on/off controlled as a function of the pulse which is modulated of the pulse width responsive to the output of the converting circuit and a transformer, whereupon the converted alternate current output is rectified and smoothed to provide a direct current constant voltage.
2. Description of the Prior Art
A variety of types of switching regulated power supply apparatuses were proposed and put into practical use. Such switching regulated power supply apparatuses may be classified depending on the type of the direct current/alternate current converting circuit, a driving system of the switching device, presence or absence of a transformer, a circuit configuration of the constant voltage control, and the like. Most typical prior art switching regulated power supply apparatuses comprise those of a chopper type, as shown in FIG. 1, and those of an opposite polarity operation converter type, as shown in FIG. 2.
More specifically, FIG. 1 shows a schematic diagram of a conventional chopper type switching regulated power supply apparatus. Referring to FIG. 1, the switching regulated power supply apparatus shown comprises a rectifier 4 coupled to receive an alternate current output from a commercial alternate current power supply 3. The output of the rectifier 4 is shunted by a capacitor 5 which serves as a smoothing circuit. The output of the rectifying and smoothing circuit is connected through a switching transistor 50 to a smoothing circuit comprising a choke coil 11 the input of which is shunted by a diode 51 and the output of which is shunted by a capacitor 12. Both terminals of the capacitor 12 are connected to the direct current output terminals 15 and 15. A potentiometer implemeted by the resistors 13 and 14 is connected across the capacitor 12. The voltage dividing terminal of the potentiometer is connected to an error detector 16 for detecting fluctuation of the voltage across the capacitor 12 with respect to a given reference voltage as an error signal, which is applied to a pulse width modulator 18. The pulse width modulator 18 is responsive to the output of a pulse oscillator 17 and the output of the error detector 16 to provide a train of high frequency pulses the pulse width of which is modulated as a function of fluctuation of the direct current output developed across the capacitor 12. The said train of high frequency pulses from the pulse modulator 18 is applied to a driver 19 and is amplified thereby. The output from the driver 19 is applied to a control electrode of the switching transistor 50 to control the conduction of the transistor 50 and thus the duty cycle of the switching control.
The FIG. 1 switching regulated power supply apparatus, however, suffers from several shortcomings. More specifically, with the FIG. 1 switching regulated power supply apparatus, only an output direct current voltage which is lower than the input direct current voltage can be obtained. Since a switching transistor is interposed between the input direct current source and the output terminal, there is a fear the short-circuiting damage of the switching transistor causes the input direct current voltage to appear at the output terminal, which threatens to damage in turn the load circuit. Since a switching transistor should be interposed between the input terminal and the output terminal, it is impossible to electrically isolate the input and output.
FIG. 2 shows a schematic diagram of a typical opposite polarity operation converter type. Referring to FIG. 2, the converter shown utilizes a high frequency transformer which is connected in series with a switching transistor. More specifically, the output of the rectifying and smoothing circuit is connected to a primary winding 60a of the transformer 60 which is connected in series with a switching transistor 61, the control electrode of which is connected to receive the output of the driver 19. The secondary winding 60b of the transformer 60 is connected to another rectifier 9, the output of which is smoothed by the smoothing capacitor 12. Since the remaining portions of the FIG. 2 converter are substantially the same as those in the FIG. 1 apparatus, it is not believed necessary to describe them in more detail.
Again, the converter shown in FIG. 2 suffers from several shortcomings. More specifically, with the FIG. 2 converter, the input direct current voltage is switched in the rectangular switching wave form. However, such a switching operation in a rectangular switching wave form is liable to cause an increased switching loss and undesired radiation. If and when the load is instantaneously short-circuited, the switching transistor comes outside a stabilized operation region of the voltage/current characteristic of the switching transistor, which threatens to damage the switching transistor. If electrical isolation between the primary and secondary windings of the transformer is improved by increasing the spacing therebetween, then a leakage inductance between the primary and the secondary windings is increased, which in turn induces ringing phenomenon in the output voltage of the transformer and causes undesired radiation and an increased switching loss of the switching transistor.
Thus, it is desired that an improved switching regulated power supply apparatus is provided wherein such shortcomings as discussed above are eliminated. The present invention achieves that purpose.